[0001] This invention relates to the entrainment of gases and particularly to the entrainment
of air in toothpaste.
[0002] Entrainment of air in a toothpaste can enhance certain properties and particularly
the appearance of a toothpaste extruded as a ribbon from a container. For example,
incorporating around 5% of air in an opaque toothpaste where the percentage of air
included is defined by the expression
density of nonaerated paste - density of aerated paste X100 density of nonaerated paste
[0003] effect a change in the appearance of the toothpaste giving it a slightly fluffy appearance.
Previous attempts to entrain between 10% and 20% air in a toothpaste packaged in a
squeeze to use container have not proved successful in that the aerated paste was
lacking in the stability necessary to give an adequate shelf life. It was found that
within one week the appearance of toothpaste extruded from a container was impaired
by the presence of large bubbles of air visible as imperfections on the surface of
the extruded ribbon due to bubble disproportionation during storage.
[0004] According to the present invention there is provided a toothpaste containing between
10 and 20% gas, the gas being incorporated as discrete bubbles having a diameter within
the range 10 to 30 micron. By providing the size distribution of the gas bubbles within
a relatively narrow range the rate of bubble dis- proportionation is substantially
reduced leading to increased stability of the packaged toothpaste. Thus the appearance
of the paste remains substantially the same and the percentage of gas contained within
the paste stays almost constant over periods of several months. Incorporating more
than 20% gas however leads to lack of stability due.to the proximity of individual
gas bubbles and hence more rapid bubble disproportionation.
[0005] Conveniently the gas can be air and the bubble size is preferably between 10 and
20 micron for long term stability.
ยท The toothpaste can be opaque toothpaste aeration of which results in a paste with
a richer more creamy appearance than the non-aerated paste, or a transparent toothpaste
aeration of which provides a pearlescent or metallised appearance.
[0006] Gas can be entrained in a toothpaste in the desired quantities and bubble sizes by
passing the material in admixture with the gas through a mixing device of the cavity-transfer
type having two closely spaced mutually displaceable surfaces each having a pattern
of cavities which overlap during movement of the surfaces so that material moved between
the surfaces traces a path through cavities alternately in each surface so that the
bulk of the material passes through the shear zone in the material generated by displacement
of the surfaces. One such mixer is described in European patent publication No 0048590A.
[0007] The invention will now be described with reference to the accompanying diagrammatic
drawing showing a plant layout for entraining air in a toothpaste.
[0008] A fully formulated toothpaste is contained within a holding vessel 1 within which
it can be subjected to low shear mixing by a stirrer 2. The vessel has an outlet line
3 connecting the bottom of the vessel to a pump 4. Between the holding vessel 1 and
pump 4 a rotameter 5 is located in the line 3 through which measured quantities of
air can be introduced into the line from inlet 6. The toothpaste is pumped from pump
4 into a cavity transfer mixer 7 and out along line 8. A two way valve 9 in the line
8 can be adjusted to return material leaving the mixer 7 along line 10 to the holding
vessel or along line 11 from which the material can be collected.
EXAMPLE 1
[0009] A plant layout as shown in the drawing was used with a cavity transfer mixer as described
in European patent publication No 0048590A. The cavity transfer mixer had an inner
rotor 5.2 cm in diameter with an effective length of 11.4 cm. The gap between the
rotor and the stator was 250 micron and the rotor speed was 300 rpm. Air was entrained
in an opaque toothpaste having a formulation based upon 50 parts by weight of aluminium
abrasive, 27 parts by weight of 70% sorbital solution and minor amounts of gum, whitener,
flavour etc. The toothpaste was passed three times through the mixer and returned
to the holding vessel until the desired amount of air was entrained within the toothpaste
before being collected from line 11.
[0010] A sample of toothpaste was examined after collection and found from photomicrographs
to have air present in bubbles of between 20 and 30 micron diameter. The percentage
of air incorporated into the toothpaste defined as
[0011] 
was determined as 11.2%.
[0012] A quantity of the toothpaste was packed into squeeze to use containers and closed
in the usual way before being stored at ambient conditions. Samples of the stored
paste were examined at intervals for appearance and percentage of entrained air. After
one month the percentage of air was unchanged at 11.2% dropping to 11.0% after lh
months and remaining constant at this level at three months. The stability ot the
toothpaste in terms of bubble disproportionation was also apparent from the substantially
unchanged appearance of a ribbon of toothpaste extruded from a container after three
months storage compared with that of the paste before storage.
[0013] It was found that the viscosity of the opaque toothpaste increased after aeration
resulting in a toothpaste with a richer more creamy appearance than the nonaerated
toothpaste. A definite increase in viscosity was observed when the bubble size was
below 30 micron.
[0014] In this case the bubbles are believed to act as solid spheres to increase the solid
phase volume and thus increase in the paste viscosity.
EXAMPLE 2
[0015] The procedure and plant of Example 1 was used to aerate a transparent toothpaste
as described in US patent specification No 3 538 230. The paste was passed once through
the cavity transfer mixer and collected from line 11.
[0016] Examination of the toothpaste showed that it contained 16% air and the majority of
gas bubbles were in the narrow range of 25 to 35 micron but with a very few larger
bubbles.
[0017] On storage in squeeze to use containers as before the percentage of air entrained
in the paste dropped after one month to 13.6% and after lk months to 12.7% and remained
at that level after 3 months. It was found however that the few large bubbles increased
in size due to bubble disproportionation during storage and these bubbles of air were
displaced from the paste and impaired the uniform appearance of the paste.
[0018] The presence of the entrained air gave the transparent toothpaste a pearlescent or
metallised appearance which remained after the toothpaste had been stored for three
months although some disproportionation of the bubbles in the 25 to 35 micron range
resulted in a reduction in the pearlescent effect.
EXAMPLE 3
[0019] The plant layout of the drawing was used with an Ystral high shear dynamic mixer
in place of the cavity transfer mixer. The toothpaste used was the same as in Example
1 and after aeration was found to contain 15.2% air and the bubble size was in the
range 10 to 100 micron with a mean diameter of 50 microns. After only 14 days the
entrained air in the paste had dropped to 12.7% and after 1 year had fallen to 10.1%.
Bubble growth due to disproportionation during the first two weeks of storage was
observed from the presence of large bubbles visible in the paste impairing the appearance
as compared to the rich and creamy appearance ot the freshly aerated toothpaste.
[0020] Thus although the aerated toothpastes of Examples 2 and 3 contained some gas bubbles
within the range of 10 to 30 micron diameter the toothpastes were unstable compared
with that of Example 1 in which the particle size distribution was within this range.
1. A toothpaste containing between 10% and 20% gas determined as hereinbefore designed,
characterised in that the gas is incorporated as discrete bubbles having a diameter
within the range 10 to 30 micron.
2. A toothpaste according to Claim 1, characterised in that the gas is air.
3. A toothpaste according to Claim 1 or Claim 2, characterised in that the bubble
size is between 10 and 20 micron.
4. A method of entraining gas in a toothpaste comprising passing the material in admixture
with the gas through a mixing device, characterised in that the gas is entrained in
the torm of discrete bubbles having a diameter within the range 10 to 30 micron with
a mixing device of the cavity-transfer type having two closely spaced mutually displaceable
surfaces each having a pattern of cavities which overlap during movement of the surfaces
so that material moved between the surfaces traces a path through the cavities alternately
in each surface whereby the bulk of the material passes through the shear zone in
the material generated by displacement of the surfaces.